1. Field of the Invention
The present invention relates to a method and an apparatus for estimating offset. More particularly, the present invention relates to a method for estimating frequency offset and timing offset implemented for channel estimation in a mobile communication system, and an apparatus thereof.
2. Description of the Related Art
A receiver device of a mobile communication system, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system, performs channel estimation in a procedure of restoring an original signal using a received signal. When the communication system employs an Orthogonal Frequency Division Multiplexing (OFDM) scheme as a standard protocol, a receiver device typically needs information such as frequency offset or timing offset to improve channel estimation performance. The receiver device may compensate for the frequency offset or the timing offset using information regarding acquired frequency offset and timing offset in a channel estimation procedure to improve channel estimation performance.
When estimating frequency offset, the receiver device performs Fast Fourier Transform (FFT) for a received signal to acquire frequency symbols, extracts pilot symbols from the acquired frequency symbols, and estimates frequency offset using a pair of pilot symbols of the extracted pilot symbols existing on the same frequency axis. The receiver device calculates a phase difference between two pilot symbols and divides the calculated phase difference by a symbol interval (namely, time interval) between two pilot symbols to estimate phase variation on a time axis for a unit symbol.
When estimating timing offset, the receiver device performs FFT for a received signal to acquire frequency symbols, extracts pilot symbols from the acquired frequency symbols, and estimates timing offset using a pair of pilots of the extracted pilot symbols existing on the same time axis. The receiver device calculates a phase difference between two pilot symbols, and divides the calculated phase difference by a frequency interval between two pilot symbols to estimate phase variation on frequency axis for each frequency.
As described above, estimation of frequency offset and estimation of timing offset are performed separately. Because pilot symbols have a lattice type pattern (referred to as ‘pilot pattern’ hereinafter) where pilot symbols are arranged to overlap each other on the same frequency axis and the same time axis (or symbol axis) in a unit resource region in an IEEE 802.16e protocol, exact offset estimation is possible due to a small quantity of errors in the offset estimation although the timing offset and the frequency offset are separately performed. In a case of a pilot pattern in the lattice form where pilots overlap each other at the same frequency and the same time as in a pilot pattern of an IEEE 802.16e communication system, the separate estimations of frequency offset and timing offset do not cause problems.
However, in a case of a pilot pattern where pilot symbols do not overlap each other at the same frequency axis and the same time axis in a Logical Resource Unit (LRU) structure as in a pilot pattern of an IEEE 802.16m system, a method for separately estimating and using frequency offset and timing offset has an ambiguity problem in estimation. When phase estimation is performed using a pair of pilot symbols distant from each other on both the time axis and the frequency axis (i.e., pilot symbols which are not located on the same frequency axis and the same time axis), because timing offset and frequency offset are estimated to overlap each other, offset estimation cannot be performed exactly. If offset estimation is not performed exactly, Mean Square Error (MSE) performance of channel estimation is degraded, and accordingly link performance is degraded in a high Signal to Noise Ratio (SNR) region.